Aluminum is produced conventionally by the electrolysis of alumina dissolved in cryolite-based (usually as NaF plus AlF3) molten electrolytes at temperatures between about 900° C. and 1000° C.; the process is known as the Hall-Heroult process. A Hall-Heroult reduction cell/“pot” typically comprises a steel shell having an insulating lining of refractory material, which in turn has a lining of carbon that contacts the molten constituents. Conductor bars connected to the negative pole of a direct current source are embedded in the carbon cathode substrate that forms the cell bottom floor. In general carbon anodes are consumed with evolution of carbon oxide gas (CO2 and CO), as gas bubbles and the like.
The consumption of carbon anodes in molten electrolyte is shown in U.S. Pat. Nos. 2,480,474 and 3,756,929 (Johnson FIG. 6a and Schmidt-Hatting et al. FIG. 1, respectively). Anodes are at least partially submerged in the bath and those anodes as well as their support structures are replaced regularly once the carbon is consumed. Alumina is fed into the bath during cell operation and it is important to have good alumina dissolution. The anode gas bubbles will help to create/cause bath flow and turbulence. It is important to create a good turbulence by anode gas bubbles to the extent favorable to increase alumina dissolution.
Traditional technology relied on natural flow of gases from under the carbon anodes during the aluminum reduction process, but this delayed gas bubble removal and decreases efficiencies and aluminum production. This presence and build up of gas generated during electrolysis has been a continuing problem in the industry and a cause of high energy requirements, and to efficiently operate the electrolysis cells, the electrodes must be properly designed.
As used to produce aluminum by the Hall-Heroult electrolytic process, there are two anode technologies. One is a pre-baked anode characterized by U.S. Pat. No. 2,480,474, mentioned previously, and U.S. Ser. No. 10/799,036, filed on Mar. 11, 2004 (Barclay et al.) The other is a “Soderberg” self-baking anode cell technology characterized by U.S. Pat. No. 3,996,117 (Graham et al.). In a pre-baked cell, there are usually 10 up to 40 anodes depending on cell size (amperage). Soderberg cells have only one large self-baking anode of approximate size, 2-3 meters wide and 5-6 meters in length. This self-baking is taught by Soderberg in U.S. Pat. No. 1,440,724.
As described by Edwards et al. in Aluminum and Its Production, MCGraw-Hill, New York, 1930, pp. 300-307, carbon anodes can be made of a mixture of carbon, pitch and tar which is pressed into molds and subsequently baked in a baking oven, or they can be made by the Soderberg technique.
In the Soderberg technique, a steel casing is used to hold carbonaceous material of electrode paste of carbon and tar-pitch. The electrode mix at the bottom end, for example in a cryolite bath, is gradually baked to provide a dense, baked carbon electrode of good conductivity, and then consumed in the cryolite by electrolysis.
As for pre-baked anodes, the use of single and multiple bottom anode slots, across the entire anode bottom, to improve gas release in aluminum processing has been reported in Light Metals, “How to Obtain Open Feeder Holes by Installing Anodes with Tracks”, B. P. Moxnes et al., Edited by B. Welch, The Minerals, Metals & Materials Society, 1998, pp. 247-255. There, 1.4 meter anodes were tested.
As shown by previously mentioned Barclay et al. U.S. Ser. No. 10/799,036 inward non-continuous slots in the bottom of a pre-baked anode can facilitate gas bubble movement and reduce energy consumption. U.S. Pat. No. 4,602,990 (Boxall et al.) taught bottom sloped either pre-baked or Soderberg anodes conforming to a sloped cathode design to either enhance or inhibit gas bubble motion However, the sloped anode can only be coupled with sloped cathodes and it cannot be used in a flat bottom cathode cell.
With their large bottom surface area Soderberg anodes can present serious problems in gas evolution. In U.S. Pat. No. 3,996,117 (Graham et al.). A carbon block anode disposed between a steel jacket provided for the upper sides of the anode is illustrated as well as anode gas, primarily CO2 bubbles, which are substantially trapped below an alumna containing crust.
In U.S. Pat. No. 5,030,335 (Olsen), the trapped CO2 gas was recognized as a problem during the passing of the CO2 gas to a disposal burner, since the gas would also contain pitch volatiles and the combustion product would have to be wet or dry cleaned. Also, breaks in the crust would allow gas escape in the furnace building. In this patent, a plurality of liftable cover plates was used as seals. In this patent, the side steel jacket/manifold for the Soderberg anode is more clearly shown. None of the previous two Soderberg cell designs solves problems of CO2 gas formation of the bottom of the anode.
In a self-baking Soderberg electrolysis cell, during electrolysis, a large quantity of anode gas (40 to 50 kg CO2/hour) is produced on the single anode bottom surface, and the anode gas has to travel a considerable distance before it can be released from the bottom surface of the anode. The gas bubbles coalesce and grow even larger before they escape from large anode bottom surface. This process of the anode gas bubble formation, coalescence, and release/escape from anode surface creates significant cell instability, and therefore, Soderberg cells usually have a lower current efficiency than pre-baked cells. At the same time, the anode gas bubbles cover a large percentage of the bottom anode surface and that results in a significant increase in electrical resistance and cell voltage, resulting in a higher energy consumption than pre-baked cell technologies.
What is needed is a Soderberg carbon anode design that will quickly channel anode gas out of the bottom horizontal surface to improve cell current efficiency, increase cell stability and reduce electrical resistance.
It is a main object of this invention to provide a cell design to reduce the amount of gas bubbles at the bottom surface of self-baking Soderberg anodes.